vSphere Virtual Disk Types

When you create a virtual machine in VMware vSphere, vSphere creates a new virtual hard
drive for that virtual machine.
The virtual hard drive is contained in a virtual machine disk (VMDK).
The disk format you choose for the new virtual hard drive can have a significant
impact on performance.

You can choose one of three formats when creating a virtual hard drive:

Thin Provisioned

Overcommitment of storage can lead to application disruption or downtime if
resources are actually used

Does not support clustering features

When vSphere creates a thin provisioned disk, it only writes a small amount of
metadata to the datastore.
It does not allocate or zero out any disk space.
At write time, vSphere first updates the allocation metadata for the VMDK, then
zeros out the block or blocks, then finally writes the data.
Because of this overhead, thin provisioned VMDKs have the lowest performance of
the three disk formats.

Thin provisioning allows you to overcommit disk spaces to VMs on a datastore.
For example, you could put 10 VMs, each with a 50 GB VMDK attached to it, on a
single 100 GB datastore, as long as the sum total of all data written by the VMs
never exceeded 100 GB.
Thin provisioning allows administrators to use space on datastores that would
otherwise be unavailable if using thick provisioning, possibly reducing costs
and administrative overhead.

Thick Provisioned Lazy Zeroed

Advantages:

Faster to provision than Thick Provisioned Eager Zeroed

Better performance than Thin Provisioned

Disadvantages:

Slightly slower to provision than Thin Provisioned

Slower performance than Thick Provisioned Eager Zero

Does not support clustering features

When vSphere creates a thick provisioned lazy zeroed disk, it allocates the
maximum size of the disk to the VMDK, but does nothing else.
At the initial access to each block, vSphere first zeros out the block, then
writes the data.
Performance of a thick provisioned lazy zeroed disk is not as good a thick
provisioned eager zero disk because of this added overhead.

By overwriting all data in the allocated space with zeros, thick provisioned
eager zeroed eliminates the possibility of reading any residual data from the
disk, thereby reducing possible security risks.

Thick provisioned eager zeroed VMDKs have the best performance.
When a write operation occurs to a thick provisioned eager zeroed disk, vSphere
writes to the disk, with none of the additional overhead required by thin
provisioned or thick provisioned lazy zeroed formats.